Miniaturized traveling wave tube

ABSTRACT

A microwave beam tube operating above 4GHz is miniaturized by designing the electron gun to produce an electron beam having a microperveance within the range of 0.8 to 2.2. A periodic permanent magnet beam focusing structure is provided which employs ring-shaped permanent magnets having a coercive force in excess of 2,000 gauss, whereby the length, size, and weight of the tube, for a given gain and output power, are reduced substantially.

Scott MINIATURIZED TRAVELING WAVE TUBE 14 1 Aug. 28, 1973 3,215,90611/1965 Taber ..315/3.5

MAGNET [75] Inventor: AllanW. Scott, Los A1to,Calif. E R d l h v R lPrimary xamineru o p o inec [73] Asslgnee' g g? Assoclates Palo AltoAssistant Examiner-Saxfield Chatmon, Jr.

Attorney-Stanley Z. Cole et a1. [22] Filed: Mar. 20, 1972 211 App].190.; 236,299 [57] ABSTRACT A microwave beam tube operating above 4GHzis min- Cl 315/535. 335/ iaturized by designing the electron gun toproduce an 335/296 electron beam having a microperveance within the [51ll?!- Cl. H01] 25/34 ran gf() 8 to 2,2, A periodic permanent magnet beamFleld of Search fogusing structure provided which employs ring- 296shaped permanent magnets having a coercive force in excess of 2,000gauss, whereby the length, size, and References Clted weight of thetube, for a given gain and output power,

UNITED STATES PATENTS are reduced substantially.

3,509,504 4/1970 Joannais 315/3.5 X 2,843,775 7/1958 Yasuda 315/35 x 7Claims 6 Drawing Figures 3,398,315 8/1968 Washbum 315/535 X 9 9 11, 13,Al 15 1115 11 ///j// Vfl/ 5 ///j/ 2 Q ///i// /W// //7// \1 )1\ (g \f/ l2 0 1 5 /j 1 1 4 ////n We: 1 k MAGNETIC E 1 FIELD f/// NOil-MAGNETICPOLEPIECE SPACER mc'mmmzam 3755706 SHEETEUFZ 20 002 20 wms) 0000 24 0BAND 2000 WATTS) g 5000- XBAND g (ZOWATTS) g 4000- CBAND SAMARIUM M1 5 2Tcoam E 3000- #25 5 00/040 3 2000- J (20 WATTS) Q I000- ALNICOBMICROPERVEANCE F|G.6 OUTPUT POWER 3 SATURATIO N 3| TPUT POWER(WATTS) l Io o 1 SATURATION I B) DESCRIPTION OF THE PRIOR ART Heretofore, periodicpermanent magnet focused traveling wave tubes operating above 4GHz havebeen built utilizing samarium cobalt ring magnets or other high coerciveforce (2,000 to 6,000 oersteds) ring magnets. However, in these priordesigns, the magnets of high coercive force were merely employed inconjunction with increased anode voltage to increase the focused beampower of the tube without redesigning the electron optics of the tube toutilize a higher perveance beam.

SUMMARY OF THE PRESENT INVENTION The principal object of the presentinvention is the provision of an improved miniaturized microwave beamtube.

In one feature of the present invention, the electron gun is designed toproduce a beam of electrons having a microperveance within the range of0.8 to 2.2. Periodic ring-shaped permanent magnets having a coerciveforce in excess of 2,000 oersteds are employed around the beam path fordeveloping a periodic beam focusing magnetic field path, such focusingfield within the beam having a peak intensity in excess of 2,000 gauss,whereby the size of the tube can be substantially decreased and theefficiency of the tube substantially improved for microwave interactionat a frequency above 4GHz.

In another feature of the present invention, the ringshaped permanentmagnets are made of a rare earth cobalt material wherein the rare earthcomponent is se lected from the group consisting of Y, Sc, La, Ce, Pr,

Nd, Sm, Eu, Gd, Eb, Dy, Ho, Er, Tm, Yb, and Lu.

In another feature of the present invention, the ring magnets have aratio of outside diameter to inside diameter falling within the range of1.3 to 3.0.

In another feature of the present invention, the microwave tube isdesigned for a power output of less than 100 watts c.w. (continuouswave).

In another feature of the present invention, the microwave energydeveloped is centered at a frequency above 8GHz.

Other features and advantages of the present invention will becomeapparent upon a perusal of the following specification taken inconnection with the accompanying drawingswh erein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic longitudinalsectional line diagram of a microwave tube incorporating features of thepresent invention,

FIG. 2 is an enlarged detailed view of a portion of the structure ofFIG. 1 delineated by line 2-2,

FIG. 3 is a plot of circuit length in inches, beam voltage in KV, andinteraction efficiency in percent, all vs. microperveance of the beamfor an X-band 20 watt c.w. traveling wave tube,

FIG. 4 is a plot of coercive force in oersteds vs. ratio of outer toinner diameter for an Alnico 8 ring-Shaped permanent magnet and asimilar magnet made of samarium cobalt,

FIG. 5 is a plot of coercive force in oersteds vs. microperveance of thebeam depicting the beam focusing ranges for Alnico 8 and samarium cobaltfor various tube designs, including a C-band traveling wave tubeoperating at various output powers, and for 20 watt c.w. traveling wavetubes at X-band and 20 GHz, and

FIG. 6 is a plot of output power in watts and saturation gain in db vs.frequency in GHz for the 20 watt c.w. X-band traveling wave tube of FIG.1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. I, thereis shown a traveling wave amplifier tube 1 incorporating features of thepresent invention. The microwave traveling wave tube 1 includes anelectron gun assembly 2 for forming and projecting a beam of electrons 3over an elongated beam path 4 to a beam collector structure 5 disposedat the terminal end of the beam path 4.

A microwave circuit 6, such as a tape helix slow wave circuit, isdisposed intermediate the gun 2 and the collector 5 along the beam path4 for cumulative electromagnetic interaction with the beam 3 foramplifying microwave energy applied to the upstream end of the microwavecircuit 6 to produce an amplified microwave signal extracted from adownstream end of the circuit 6 by a suitable output RF coupling means.

A conventional evacuated envelope structure 7, shown in dotted lines,encloses the electron gun, slow wave circuit 6 and collector 5. Aperiodic permanent magnet beam focus structure 8 is disposed surroundingthe beam path 4 intermediate the gun 2 and collector 5 for focusing theelectron beam 3 through the microwave circuit 6.

Referring now to FIG. 2, the beam focus magnetic structure 8 is shown ingreater detail. The periodic permanent magnet focus structure 8 includesa series of ring-shaped permanent magnets 9 made of a permanent magnetmaterial having a coercive force preferably higher than 2,000 oersteds.Examples of such permanent magnet material includes the rare earthcobalt magnet materials wherein the rare earth component of the magnetis selected from the group consisting of Y, Sc, La, Ce, Pr, Nb, Sm, Eu,Gd, Tb, Dy, Ho, Er, Pm, Yb, and Lu. Such rare earth cobalt magnetmaterials are disclosed and claimed in copending US. application Ser.No. l64,066 of L. R, Falce filed July 19, I971 and assigned to the sameassignee as the present invention. In ring-shaped magnets the peakmagnetic field intensity H, produced on the axis of the magnet, forfocusing the beam 3, is approximately equal to the coercive force of themagnet material.

Adjacent ring magnets 9 are spaced by Y magnetic pole plates 11, as ofiron, having inner annular flange portions 12. The pole pieces 11 arespaced apart by metallic annular non-magnetic spacers 13. The annularpole pieces 11 and spacers 13 are brazed together at their adjoiningfaces to fonn the hollow cylindrical vacuum tight envelope 7 of thetube 1. The ring-shaped magnets 9 are preferably axially cut to form twohalf ring members which are then slipped over the envelope 7 andretained, by retaining rings, not shown, between the pole pieces 11.

The helical slow wave circuit 6, not shown in FIG. 2,.

is supported in the conventional manner via three longitudinallydirected ceramic support rods, as of boron nitrate, spaced at intervalsaround the outside of the helix. An interference fit is obtained betweenthe helix, the helix support rods, and the inside cylindrical bore ofthe envelope 7, formed by the inside surface of the annular pole pieceflanges I2 and the non-magnetic spacer 13 to facilitate thermalconduction from the helix 6 to the envelope 7.

Referring again to FIG. 1, the electron gun 2 is designed with amicroperveance in the range of 0.8 to 2.2, where microperveance isdefined as the beam current divided by the 3/2 power of the beamvoltage. Microperveance is a convenient parameter because the requiredmagnetic focus field is proportional to the square root of themicroperveance. In a typical example, the microperveance of the electrongun 2 is 1.0, the area convergence of the spherically concave cathodeemitter 14 and anode is 13.5 to 1, which gives a cathode loading of 2amps per square centimeter. The cathode emitter 14 is preferably adispenser cathode. The diameter of the cathode button for a 20 watt X-band tube is 0.0845 inch and its spherical radius is 0.086 inch.

The electron gun 2 includes a centrally apertured anode 15 through whichthe electron beam 3 is projected into the region of the slow wavecircuit 6 and periodic permanent magnet structure 8. The electron gun 2has a modulating anode 16 such that the tube 1 can be operated underdual mode conditions, i.e., pulsed or c.w. The cathode l4, modulatinganode l6, and anode 15 are stacked between ceramic cylinders, not shown,of 0.342 inch OD. and 0.254 ID. to reduce the diameter of the gun 2 tobe compatable with the miniature periodic permanent magnet structure 8.

Heretofore, 20 watt X-band traveling wave tubes have been designedemploying Alnico 8 ring-shaped permanent magnets for focusing the beam.Alnico'8 has a maximum coercive force of approximately 1,500 oersteds;this limits the design of the gun to approximately a microperveance of0.6 as shown in FIG. 5. As a consequence, the prior art tube had arelatively low efficiency (on the order of 12 percent, as indicated inFIG. 3), required a beam voltage of approximately 2.5 KV, and had anoverall circuit length of approximately 3.75 inches.

In the present invention, a higher coercive force magnet material isutilized, such as samarium cobalt, which has the coercive force inoersteds vs. ratio of outer to inner magnet diameter d ld, shown bycurve 21 of FIG. 4. By selecting the ratio of outer diameter to innerdiameter of the magnetic rings 9 to be approximately 1.5, a coerciveforce in oersteds of approximately 3,000 is obtained. When stabilizedfor thermal effects at 2,500 oersteds, this yields a peak axial magneticbeam focusing field intensity within the beam of approximately 2,250gauss. This then permits a gun microperveance of 1.0 for an X-band 20watt tube as shown by curve 22 of FIG. 5.

By utilizing a microperveance of 1.0, the beam voltage is reduced toapproximately 1,750 volts, the circuit length is reduced toapproximately 2.3 inches, and the efficiency of the tube is increased toapproximately 15 percent. However, by designing the electron gun 2 witha higher microperveance of 2.0, the beam voltage can be reduced toapproximately 1,250 volts, the circuit length reduced to approximately1.5 inches, and the efficiency increased to approximately 20 percent foran X-band traveling wave tube operating with a power output ofapproximately 20 watt c.w. and employing periodic permanent magnetshaving a coercive force of approximately 5,500 oersteds as shown inFIGS. 3 and Thus, the use of rare earth cobalt magnets, having arelatively high coercive force as illustrated by samarium cobalt in FIG.5, permits use of tubes utilizing electron guns having microperveancesin the range of 1.0 to 2.0. This allows either increasing the poweroutput of the tube at a given frequency, such as C-band, as indicated bythe C-band curves 22, 23 and 24 or the de sign of microwave tubesutilizing a gun 2 having a microperveance in the range of 1.0 to 2.0 athigher frequency ranges than were heretofore achievable. For example,Alnico 8 could not have been used for focusing a 1.0 to 2.0microperveance electron beam for a 20 watt tube at X-band. The maximummicroperveance that could be employed with such a magnetic material was0.6.

Since the magnetic field required to focus an electron beam dependsdirectly on the square root of the beam microperveance and inversely onthe beam diameter, the greater the microperveance, the greater therequired magnetic field. Since beam diameter is inversely proportionalto the frequency of the tube, the required focusing field increasesdirectly with frequency. The higher the frequency range of the tube, thegreater the required focusing field for a given microperveance.

This is roughly illustrated by curves 22, 23 and 24 of FIG. 5 where theC-band curve for 2,000 watts corresponds roughly to the curve for a20GHz tube at 20 watts c.w. An X-band tube to produce 20 watts c.w.corresponds roughly to a C-band tube producing 200 watts c.w., and aC-band 20 watt curve corresponds roughly to an X-band 2 watt curve.

In a 20 watt X-band tube of the present invention, the outputperformance of the tube isshown in FIG. 6. The c.w. power output, asillustrated by curve 30, is greater than 20 watts over the range of 8 tol2GHz and the saturated gain, curve 31, is greater than 40db over therange of 8 to I2GI-Iz. The tube has an overall size that permits it tofit inside a standard Xband waveguide so that the outer diameter of themagnets was less than 0.4 inch; the magnet outer to inner diameter ratiois 1.5, with the magnet coercive force being 2,500 oersteds aftertemperature stabilization. This permitted focusing of a beam with amicroperveance of 1.0. The weight of the tube, with the focusing magnet,was 3 ounces; its

overall size was 0.35 inch by 0.75 inch by 5.0 inches.

The tube had an overall efficiency including heater power of 20 percent.

The miniaturization of medium power traveling wave tubes by the use ofrare earth cobalt magnets, such as samarium cobalt, is applicablethroughout the C, X and Ku-band frequency ranges. At lower frequencies,conventional magnets have always been strong enough, and existing tubeshave already achieved their minimum size.

Octave bandwidth is readily available from the miniaturized tubes of thepresent invention. Miniaturization can be effectively applied at c.w.power levels from milliwatts up to 50 watts and at peak power levels upto a few hundred watts.

Although the present invention has been described thus far for focusingthe beam of a traveling wave tube, it is also applicable in general toother linear beam tubes, such as backward wave oscillators, klystrons,and the like.

What is claimed is:

1. In a miniaturized periodic permanent magnet focused microwave beamtube, means for forming and projecting a beam of electrons of amicroperveance within the range of 0.8 to 2.2 over an elongated beampath, microwave circuit means disposed along said beam path in microwaveenergy exchanging relation with the beam for cumulative electroninteraction with the beam to develop microwave energy on said microwavecircuit, such developed microwave energy being of a frequency above4Gl-lz, periodic ring-shaped axially polarized permanent magnet meanshaving a coercive force in excess of 2,000 oersteds and disposedcoaxially of and along said beam path for developing a periodic magneticbeam focusing field of an intensity within the beam path in excess of2,000 gauss for focusing the electron beam through said microwavecircuit.

2. The apparatus of claim 1 wherein said microwave circuit means is aslow wave circuit.

3. The apparatus of claim 1 wherein said ring-shaped permanent magnetsare made of a rare earth cobalt material wherein the rare earth materialcomponent is selected from the group consisting of, Y, Sc, La, Ce, Pr,Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu.

4. The apparatus of claim 1 wherein said ring magnets have a ratio ofouter diameter to inner diameter falling in the range between 1.3 and3.0.

5. The apparatus of claim 1 including means for extracting the developedmicrowave energy from said microwave circuit as output microwave power,and wherein the output microwave power is in a range less than wattsc.w.

6. The apparatus of claim 1 wherein said microwave energy developed onsaid circuit is centered at a frequency above 8GHz.

7. The apparatus of claim 1 wherein said microwave circuit is a helixslow wave circuit.

1. In a miniaturized periodic permanent magnet focused microwave beamtube, means for forming and projecting a beam of electrons of amicroperveance within the range of 0.8 to 2.2 over an elongated beampath, microwave circuit means disposed along said beam path in microwaveenergy exchanging relation with the beam for cumulative electroninteraction with the beam to develop microwave energy on said microwavecircuit, such developed microwave energy being of a frequency above4GHz, periodic ringshaped axially polarized permanent magnet meanshaving a coercive force in excess of 2,000 oersteds and disposedcoaxially of and along said beam path for developing a periodic magneticbeam focusing field of an intensity within the beam path in excess of2,000 gauss for focusing the electron beam through said microwavecircuit.
 2. The apparatus of claim 1 wherein said microwave circuitmeans is a slow wave circuit.
 3. The apparatus of claim 1 wherein saidring-shaped permanent magnets are made of a rare earth cobalt materialwherein the rare earth material component is selected from the groupconsisting of, Y, Sc, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm,Yb, and Lu.
 4. The apparatus of claim 1 wherein said ring magnets have aratio of outer diameter to inner diameter falling in the range between1.3 and 3.0.
 5. The apparatus of claim 1 including means for extractingthe developed microwave energy from said microwave circuit as outputmicrowave power, and wherein the output microwave power is in a rangeless than 100 watts c.w.
 6. The apparatus of claim 1 wherein saidmicrowave energy developed on said circuit is centered at a frequencyabove 8GHz.
 7. The apparatus of claim 1 wherein said microwave circuitis a helix slow wave circuit.